1. Field of the Invention
The present invention relates to sizing agents and more particularly it relates to the sizing of cellulose fibers and wood chips with emulsions of ketene dimers and nonreactive hydrophobes.
2. Description of the Prior Art
Building products such as particle board and wafer board are made from mechanically chipped wood that is not digested and not bleached. Products such as paper, paperboard, and medium density fiberboard are made from cellulosic fibers that are produced by digestion of chipped wood. The cellulosic fibers may be bleached before being made into these products.
Cellulosic fibers and wood chips may need to be made to repel liquids in order to be successful in their final forms. Commonly, they need to repel inks or water, i.e. the fibers and wood chips need to be sized.
Over the years many types of sizing agent have been employed. The common sizing agents are based on rosins, ketene dimers, waxes, silicone resins or fatty acid derivatives. Each sizing agent has its advantages and disadvantages. Each sizing agent may advantageously be used under different conditions. The present invention relates to the sizing of cellulosic fibers and wood chips under neutral or alkaline conditions.
It is well known that alkyl (aryl, etc.) ketene dimers (AKD's) can be used successfully to size bleached pulp. AKD's are water insoluble and are therefore generally supplied as aqueous emulsions. These emulsions tend to be unstable, and it is therefore common practice to include stabilizers, such as starch derivatives, in particular cationic starch derivatives, in such emulsions. In this respect, reference is made for instance to U.S. Pat. No. 3,130,118 and U.S. Pat. No. 2,627,477. AKD's act by chemically reacting with the wood fibers in the pulp. In some circumstances the reaction can be relatively slow and therefore, in order to speed up the reaction, promoter resins can be added to AKD emulsions. Promoted AKD emulsions are available commercially.
Although in many applications AKD's are very useful in sizing pulp, they have certain drawbacks. In particular, AKD's work better with promoter resins in order to effectively size unbleached pulp. It has been conjectured that the reason for this is that the lignins and other chemical contaminants in unbleached pulp interfere with the reaction between the AKD and the fiber so that effective sizing is prevented.
Promoted AKD's are generally more effective in applications where a significant level of sizing is required before a size press, or before an on-machine coater or before an off-machine coater.
An alternative to AKD sizing in such applications is the use of nonreactive hydrophobes, such as waxes, hydrocarbon resins or terpene resins, added to the bleached or unbleached pulp, to give sizing in the body of the product, or used as a surface sizing agent. Hydrophobes such as wax are used also with wood chips in the manufacture of building products. The nonreactive hydrophobe does not react with the fibers or wood chips, but is present merely as a water repellent deposit on the fiber or wood chips. In order to obtain good sizing, a relatively large amount of nonreactive hydrophobe is required. However, this brings with it the disadvantage that the presence of the large quantity of nonreactive hydrophobe reduces the strength of the product. It is believed that this is because the hydrophobe interferes with the fiber interactions which contribute to the strength of the product.
This is of particular importance in building products where resins are added as binding agents. In these products, the extract pH may be approximately 5.5. The use of wax as the hydrophobe increases the slipperiness of the product, which is an advantage in the manufacture of building products, but is a disadvantage in paper and paperboard products.
A further problem with the use of wax as a sizing agent is that wax emulsions are generally unstable. There have therefore been a number of proposals for improving the stability of wax emulsions, for instance by the addition of emulsifying agents. In this respect, reference can be made, for instance, to U.S. Pat. No. 3,096,232.
The problem of wax emulsion stability is also addressed in U.S. Pat. No. 4,017,431. This patent discloses a stable aqueous wax emulsion. The stability of the emulsion is enhanced by blending either a fatty acid or an AKD with the wax and then using the blend to form a homogeneous dispersion of the blend in an aqueous medium. The medium preferably includes a stabilizing resin comprising an epoxy-terminated aminopolyamide. The patent shows the use of such homogeneous emulsions in the sizing of bleached pulp. The results given show an improvement in emulsion stability and a linear increase in sizing with increase in the AKD content of the blend. In view of the known adverse effect of lignins etc. on AKD sizing, it would be expected that the only part of such a blend which would be effective in sizing unbleached pulp would be the wax.
Japanese patent application J53 087395 (Kao Soap) also addresses the problem of slow development of sizing with AKD's by proposing mixtures containing AKD's and waxes dispersed together in water. Mixtures having ratios between 5-200 parts wax to 100 parts AKD, preferably 10-100 parts wax to 100 parts AKD, are disclsoed. Example 3 of this application shows that the rate of sizing development reaches a maximum at a ratio of about 5-10 parts wax to 100 parts AKD and that higher ratios of wax up to 100 parts wax to 100 parts AKD no not increase further the rate of development of sizing. This same Example shows also that the final level of sizing achieved after 1 day with these mixtures is not significantly higher than the level of sizing achieved by AKD alone.
It is an aim of the present invention to provide an improved method of and composition for sizing unbleached fibers for use in applications such as medium density fiberboard; and unbleached, undigested wood chips for use in the manufacture of particle board, wafer board and the like building products.